1. Field of the Invention
The present invention generally relates to nuclear medicine, and systems for obtaining nuclear medical images of a patient's body organs of interest. In particular, the present invention relates to a novel collimator with variable focusing and direction of view for nuclear medicine imaging, particularly for single photon imaging including single photon emission computed tomography (SPECT).
2. Description of the Background Art
Nuclear medicine is a unique medical specialty wherein radiation is used to acquire images that show the function and anatomy of organs, bones or tissues of the body. Radio pharmaceuticals are introduced into the body, either by injection or ingestion, and are attracted to specific organs, bones or tissues of interest. Such radio pharmaceuticals produce gamma photon emissions that emanate from the body. One or more detectors are used to detect the emitted gamma photons, and the information collected from the detector(s) is processed to calculate the position of origin of the emitted photon from the source (i.e., the body organ or tissue under study). The accumulation of a large number of emitted gamma positions allows an image of the organ or tissue under study to be displayed.
Single photon imaging, either planar or SPECT, relies on the use of a collimator placed between the source and a scintillation crystal or solid state detector, to allow only gamma rays aligned with the holes of the collimator to pass through to the detector, thus inferring the line on which the gamma emission is assumed to have occurred. Single photon imaging techniques require gamma ray detectors that calculate and store both the position of the detected gamma ray and its energy.
Two principal types of collimators have been used in nuclear medical imaging. The predominant type of collimation is the parallel-hole collimator. This type of collimator contains hundreds of parallel holes, which can be formed by casting, drilling, or etching of a very dense material such as lead. Parallel-hole collimators are most commonly attached near the detector (scintillator) with holes arranged perpendicular to its surface. Consequently, the camera detects only photons traveling nearly perpendicular to the scintillator surface, and produces a planar image of the same size as the source object. In general, the resolution of the parallel-hole collimator increases as the holes are made smaller in diameter and longer in length. The parallel-hole collimator offers greater sensitivity than a pinhole collimator, and its sensitivity does not depend on how closely centered the object is to the detector.
The conventional pinhole collimator typically is cone-shaped and has a single small hole drilled in the center of the collimator material. The pinhole collimator generates a magnified image of an object in accordance with its acceptance angle, and is primarily used in studying small organs such as the thyroid or localized objects such as a joint. The pinhole collimator must be placed at a very small distance from the object being imaged in order to achieve acceptable image quality. The pinhole collimator offers the benefit of high magnification of a single object, but loses resolution and sensitivity as the field of view (FOV) gets wider and the object is farther away from the pinhole.
Other known types of collimators include converging and diverging collimators. The converging collimator has holes that are not parallel; rather, the holes are focused toward the organ with the focal point being located in the center of the FOV. The image appears larger at the face of the scintillator using a converging collimator. For equivalent spatial resolution the converging collimator has higher sensitivity than the parallel-hole collimator. The gain in point sensitivity is obtained at the price of a reduced FOV. The diverging collimator results by reversing the direction of the converging collimator. The diverging collimator is typically used to enlarge the FOV, such as would be necessary with a portable camera having a small scintillator. The diverging collimator has a lower sensitivity than the parallel-hole collimator, especially with thick objects.
Another type of collimator is slat collimator that has been used with a rotating laminar emission camera, also known as the rotating laminar radionuclide camera. This camera has linear collimators usually formed by mounting parallel collimating plates or slats between a line of individual detectors. Alternately, individual detector areas of a large-area detector are defined and isolated through the placement of slats. The slat collimator isolates planar spatial projections; whereas, the grid collimator of traditional scintillation detectors isolates essentially linear spatial projections. The detector-collimator assembly of a slat camera is typically rotated about an axis perpendicular to the detector face in order to resolve data for accurate two-dimensional image projection. The projection data collected at angular orientations around the subject are reconstructed into a three-dimensional volume image representation.
While maintaining certain advantages, such as a better sensitivity-resolution compromise, over, e.g., traditional Anger cameras, slat detectors are burdened by some other undesirable limitations. For example, the one dimensional collimation or slat geometry used by slat detectors complicates the image reconstruction process. The slat geometry results in a plane integral reconstruction as opposed to the line integral reconstruction that is generally encountered in traditional Anger camera applications. Moreover, the geometry produces a plane integral only in a first approximation.
It is well known in the art that nuclear medicine imaging of small organs, such as brain, heart, kidneys, thyroid, and the like present special problems in collecting radiation emission and creating images from the collected data. Different systems including the use of the above described collimators have been used for nuclear imaging of small organs. Although images of such organs are routinely made, there remains a need for a system and methodology for improving imaging of small organs and for overcoming the shortcomings of the prior art, such as a novel collimator for a nuclear imaging camera and a method of forming the same.